In a typical fossil-fired boiler, tube outlet steam temperatures and tube metal temperatures are not uniform throughout the tube circuits. While the bulk steam temperature at the tube circuit outlet header may typically be 1005.degree. F., the local steam temperatures in some of the tubes can be as much as 150.degree. F. higher or lower than the bulk temperature. These temperature variations typically occur both across the tube circuit from left to right and through each tube assembly in the direction of the gas flow. The cause of these variations is typically a combination of nonuniform gas velocity and temperature distributions, steam flow imbalance, and intrinsic characteristics of convection pass heat transfer surface arrangements. In general, boiler manufacturers attempt to account for these temperature variations by specifying tube and header materials and thicknesses based upon worst case design conditions.
Under actual operating conditions, a nonuniform tube metal temperature distribution can often lead to metal temperatures in excess of the worst case design in localized areas of the tube circuit. This is generally due to off-design operating conditions, changes from design fuel, and errors in design. These elevated metal temperatures cause tube failures due to high temperature creep. In addition, several other problems are created, such as increased thermal strains that result in header bowing and ligament cracking with premature failures in the associated header components. Decreased thermal performance, boiler efficiency, and reduced life also result.
These undesirable factors have been accepted as typical of operation and characteristic of design. For example, boilers with a tangential firing pattern are usually hotter on one side of the superheater and reheater sections. Front and rear wall fired boilers typically have hot spots at the quarter points on the header. These temperatures are the result of gas side and steam side flow imbalances occurring across the unit that are partially addressed in the original design calculations. However, the reality of the large temperature differences is that tube materials and header geometry have generally not been adequately designed to withstand these differences. For example, material changes are made in a circuit from the inlet to the outlet, but the same materials are used across the unit. Each assembly across a unit is identical even though temperature differences can vary by as much as 150.degree. F. This temperature difference is almost as large as the temperature difference from the inlet to the outlet in a particular tube assembly.
Failures of boiler tubes due to high temperature creep are a leading cause of forced outages in fossil fueled boilers. Often these failures are confined to very localized regions of the tube circuit for the reasons cited above. Furthermore, when the tube failure frequency becomes unacceptably high for the utility, the entire tube circuit is often replaced when, in fact, only a small region of the tube circuit has significant creep damage and the remainder of the tube circuit has substantial remaining life.
FIG. 1 shows a typical profile of the steam temperature at the tube outlet legs of a superheater situated in a fossil fueled boiler. These temperatures were obtained from thermocouples welded to the outside of tubes just upstream of the outlet header. Since there is negligible heat flux in this region, this measured temperature is indicative of both metal temperature and steam temperature at the tube outlet. Note that in the center of the superheater, steam temperatures are substantially higher than the design bulk steam temperature of 1005.degree. F., while at either side of the superheater, the steam temperature is substantially below this value.
Clearly, in the example of FIG. 1, the center tubes are running hotter than the outside tubes. If this is typical of the unit operation from the beginning, then the center tubes will have substantially less remaining creep life than the outside tubes. Also, it is pointed out that tube metal temperatures in the furnace section where a heat flux is imposed on the tube will be even higher than the outlet steam temperatures in FIG. 1.
FIG. 2a shows the creep damage accumulation rate of a typical boiler tube throughout its life. At an operating time of 200,000 hours, slightly over eighty per cent of the creep life of the tube has been consumed. If the tube continues to operate under the same temperature conditions, it will fail due to creep at approximately 225,000 hours.
FIG. 2b expands the upper portion of the curve of FIG. 2a. It can be seen that if the temperature of this tube could be lowered at the 200,000 hour point, then its remaining life could be significantly extended. For instance, by lowering the temperature 30.degree. F., the remaining life would be extended from 25,000 to 75,000 hours. Each tube will have its own unique life gain depending on when and how much its temperature is reduced, how fast creep damage is accumulating, how much original life remains, and the wall thinning rate due to fireside erosion. These unique curves illustrate the benefit which can be derived according to the present invention.